The disclosures herein relate generally to heat transfer and more particularly to a thermally activated device for coupling and decoupling heat sinks, especially those used in electronic devices contained in computer systems.
A portable computer is a self-contained personal computer which can be easily moved to and operated at various locations. Portable computers are often referred to as laptop or notebook computers. To be portable, these computers must be small, compact, and lightweight. The conventional portable computer includes a base portion and a lid portion that pivotally opens from the base portion when the portable computer is in use. The lid portion contains a flat panel display such as a liquid crystal display (LCD) or other relatively small display. Notebook computers incorporate electromagnetic shielding to reduce electromagnetic emissions in order to meet regulatory standards on such emissions.
Portable computers also use thermal transfer mechanisms to cool the central processing unit (CPU) which accounts for as much as half of the power dissipated within the computer system and other thermally sensitive components such as PC cards and SDRAM. In the past, heat spreaders, heat sinks, heat pipes, and fans have been used to address the heat transfer problem. One type of heat spreader is a metal piece that is thermally coupled to a processor and distributes heat away from the processor. Typically, a heat sink is made of relatively pure aluminum for good thermal conductivity and for reduced weight. However, because aluminum is a relatively soft metal with a low yield strength, it makes for a poor electromagnetic shield. Furthermore, oxide coatings typically form on the outside of items made of aluminum which reduces the ability of the object to provide low impedance electrical connections with other items in contact with the aluminum material. Because heat spreaders typically serve only one function, heat spreaders add extra pieces to the computer system assembly as well as increasing the complexity of the build and repair operations.
Fans and heat sinks provide a cost effective mechanism for thermally managing many types of portable computer systems. Fans, however, require power and heat sinks require space. While power and space are generally in abundant supply in desktop-type minicomputers, portable computers have only a limited supply of both power and space. A commercial advantage is achieved by manufacturing portable computers that are both small and lightweight. Further, portable computers must operate with power conservation in mind. An operable fan may unduly draw upon the batteries of a laptop making it unattractive for long periods of battery-operated use.
Heat pipes are self contained, phase transformation, heat carrying devices, i.e. a superconductor of heat. A typical heat pipe may comprise a closed copper tube having a partial vacuum internally. Water in a hot portion of the tube boils at a lower than usual temperature in the partial vacuum. The boiling water seeks a cooler spot and thus steam moves to carry heat to the cooler spot where the steam condenses to cooler water which returns to the hot spot. The cycle is ongoing which provides a self-contained circulating cooling system.
Computer chassis contain many heat producing components. The heat must be removed from the computer chassis. Some of these components can operate adequately at higher temperatures than others. Therefore, it is not necessary to remove the same quantity of heat from all components. For example, a PC card may have an operating limit of 55.degree. C. at 3 watts, and a CPU may have an operating limit of 100.degree. C. at 12 watts. Also, a palm rest area of a portable computer may become uncomfortable to the touch of the operator if the temperature of the palm rest exceeds 45.degree. C.
A bimetallic heat spreader has been described in U.S. Pat. No. 5,469,329. In this device, semiconductor dies and other electronic devices may be mounted directly on a heat spreader made of copper so that the electronic devices are capable of transmitting heat directly to the copper, although they are indirectly supported by the circuit board that serves as a carrier for the electronic devices. The heat may be transferred directly from the electronic devices to a heat sink through the heat spreader. Thermal resistance in the transfer of thermal energy to the heat sink thus is minimized.
Differences in the coefficients of thermal expansion of the copper of the heat spreader and the substrate of the electronic device, such as a power die, are accounted for by using a metal layer of Invar within the heat spreader in the localized area on which the electronic device is mounted. The Invar, which is characterized by very low thermal conductivity, does not interfere with the heat transfer because it is remote from the heat flow path. The silicon die, which is a poor thermally conductive material having a low coefficient of thermal expansion, then can be bonded to the localized area of the copper heat spreader without being adversely affected by differences in the thermal coefficients of expansion of the copper and the silicon. The bonding may involve the use of solder metal or conductive epoxies.
A secure bond can be achieved between the die and the heat spreader without resorting to conventional plastic encapsulating techniques that have been used in the manufacture of prior art wiring board assemblies to minimize a thermal mismatch between the heat spreader and the die. Thus, the die or other electronic device that is attached to the heat spreader can be operated at a higher power level without adversely affecting its reliability due to the presence of operating temperatures within safe operating junction temperature limits.
The heat spreader itself can be attached to the printed wiring board at pre-cut locations and made an integral part of the board using conventional soldering techniques. The printed wiring board material that defines the electrical conduits, therefore, is situated remotely from the heat conduction path of the heat spreader as thermal energy is transferred from the electronic device through the heat spreader into a heat sink.
Therefore, what is needed is an apparatus and a method providing thermally activated coupling and decoupling multiple heat sinks of the type used in computer systems.